1. Field of the Invention
The present invention relates to a controlling apparatus and a controlling method for controlling a wavelength division multiplexing (WDM) optical amplifier for collectively amplifying wavelength division multiplexed signal light beams to be used in optical communications, and particularly to a controlling apparatus and a controlling method for a wavelength division multiplexing optical amplifier, capable of reducing errors due to the affection of noise light to thereby realize a controlling operation with high precision.
2. Description of the Related Art
Attention has been recently directed to an optical wavelength division multiplexing (WDM) transmission system, as a technique adapted to drastically increase a transmission capacity per one thread of optical fiber transmission path, and as a basic technique leading to a lightwave network constitution. On the other hand, optical amplifiers are able to linearly amplify optical signals without once converting them into electrical signals, to thereby largely improve regenerating and repeating intervals. Thus, optical amplifiers are widely used particularly in a main line transmission system. Further, since the optical wavelength division multiplexing transmission system is also able to collectively amplify a plurality of optical signals, so that the advantage of optical amplifiers can be effectively utilized.
In a conventional optical transmission system adopting optical amplifiers, it is impossible to obtain transmission channel information by monitoring such as overheads of electrical signals at an optical amplifying-repeating device (linear repeating station). Thus, as alternative means therefor, there is known a method for wavelength division multiplexing optical supervisory channel signal light (hereinafter called “OSC signal light”) into main signal light. This OSC signal light is generated such as by a WDM terminal equipment at a transmission side and terminated at an optical amplifying-repeating device, and necessary information is again multiplexed into the OSC signal light, to be transferred to a WDM terminal equipment at a receiving side.
Level controlling systems for optical amplifiers to be used in the aforementioned optical transmission system include one for keeping constant the output level of an optical amplifier (hereinafter called “ALC system”) and another for keeping constant the gain (a difference between input and output levels) of an optical amplifier (hereinafter called “AGC system”).
FIG. 10 is a block diagram showing an exemplary constitution of a general ALC system. In the ALC system such as shown in FIG. 10, there is monitored a portion of output signal light output from light level controlling means including such as an optical amplifier and an optical attenuator, and such as the gain of the optical amplifier and the loss of the optical attenuator are feedback controlled so as to keep constant the averaged level of the output signal light.
FIG. 11 is a block diagram showing an exemplary constitution of a general AGC system. In the AGC system such as shown in FIG. 11, there are monitored a portion of input signal light into light level controlling means and a portion of output signal light output from light level controlling means, and the light level controlling means is feedback controlled so as to keep constant the difference between the averaged levels of the input light and output light.
In case of utilizing an optical amplifier in optical wavelength division multiplexing transmission, when the number of wavelength channels is changed while keeping a reference signal in FIG. 10 at a constant value under the control of ALC system, there is caused fluctuation in wavelength channel levels as shown in FIG. 12. Namely, if the number of wavelength channels is changed to 4 channels when the system is ALC operating with a setting that the number of wavelength channels is 2 channels, the light level controlling means is feedback controlled such that the averaged level of the whole of the output signal light at 2 channels and that at 4 channels are constant because the reference signal of a comparator is constant, resulting in the respective wavelength channel levels being lowered after changing the number of wavelength channels. As such, it is also necessary in the ALC system to change the reference signal corresponding to the number of wavelength channels being used. In this respect, it is possible to obtain the information about the number of wavelength channels being used, such as via the OSC signal light transmitted from the WDM terminal equipment at the transmission side to the optical amplifying-repeating device.
Meanwhile, when the optical amplifier is AGC operating, the levels of respective wavelength channels are kept constant as shown in FIG. 13 even if the number of wavelength channels is changed. Namely, since the difference between the averaged levels of the input light and output light is kept constant even if the number of wavelength channels is changed from 2 channels to 4 channels, the respective wavelength channel levels are controlled to be constant before and after the change of the number of wavelength channels.
However, in the conventional controlling method for an optical amplifier for optical wavelength division multiplexing as described above, there has existed a problem of a controlling error due to the affection of noise light called amplified spontaneous emission (ASE) light to be caused upon linearly amplifying optical signals.
FIG. 14 shows an example of a spectrum of an optical wavelength division multiplexed transmission signal, where the number of wavelength channels is 8 channels. In FIG. 14, what can be seen at the lower side of spectrum peaks of respective wavelength channels, is ASE light caused upon linearly amplifying optical signals.
The spectrum shape of ASE light is determined by the amplification band of an optical amplifier, optical amplifiers having wider amplification bands (i.e., capable of amplification of many wavelength channels) have wider noise bands, and the ASE light is accumulated whenever the linear amplification is repeated. Concerning the optical signal to be monitored when conducting the level control of an optical amplifier, the light powers of respective wavelength channels are not individually monitored, but the whole of the light output of the optical amplifier is monitored such as by a PD (photodetector). This results in the inclusion of the aforementioned ASE light, leading to an error upon conducting the level control of wavelength channel light. The controlling error due to ASE light becomes more apparent, when an optical amplifier having a wide band is to be used at a smaller number of wavelength channels, for example, when an optical amplifier having a wavelength band corresponding to 32 channels is used at 1 channel.
Further, the purpose of the ALC operation of a WDM optical amplifier is to suppress the drift of an input signal level such as due to the loss fluctuation of a transmission path, to thereby ensure a stable transmission quality. As such, it is also desired to ALC operate the optical amplifier when a wavelength channel(s) is(are) added or subtracted. However, in the conventional controlling method, it has been required to once stop the ALC, when adding/subtracting wavelength channel(s).
Further, when an optical amplifier is AGC operating, it is required that the amplitude and speed of level fluctuation to be caused by addition or subtraction of wavelength channels are within a range of the AGC operation response, such that the change of the number of wavelength channels does not affect the light levels of respective wavelength channel lights. However, the response speed of AGC by the conventional controlling method has not been necessarily at a sufficient level.
Still referring to FIG. 14, the ratio between the level of wavelength channel light and that of the noise light is called an optical signal to noise ratio (hereinafter called OSNR) which is an important parameter in evaluating the quality of optical transmission signals. To accurately measure the OSNR, it is necessary to measure the spectrum of optical signals, which requires a spectrum analyzer having a wide dynamic range. However, from the standpoint of simplification of the constitution of an optical amplifying-repeating device, it is not necessarily advisable to provide a spectrum analyzer in each optical amplifying-repeating device for the purpose of measuring the OSNR.